The present invention relates to magnetoresistor devices used for magnetic position sensors.
The use of magnetoresistors (MRs) and Hall devices as position sensors is well known in the art. For example, a magnetically biased differential MR sensor may be used to sense angular position of a rotating toothed wheel, as for example exemplified by U.S. Pat. Nos. 4,835,467, 5,731,702, and 5,74,042.
In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited. typically. with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object moving relative and in close proximity to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the moving target wheel is adjacent to the MR than when a slot of the moving target wheel is adjacent to the MR.
Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,731,702, and 5,754,042).
The shortcoming of MR devices is their temperature sensitivity. They have a negative temperature coefficient of resistance and their resistance can drop as much as 50% when heated to 180 degrees Celsius. Generally, this led to the use of MR devices in matched pairs for temperature compensation. Additionally, it is preferable to drive MR devices with current sources since, with the same available power supply, the output signal is nearly doubled in comparison with a constant voltage source.
To compensate for the MR resistance drop at higher temperatures, and thus, the magnitude decrease of the output signal resulting in decreased sensitivity of the MR device, it is also desirable to make the current of the current source automatically increase with the MR temperature increase. This is shown in U.S. Pat. No. 5,404,102 in which an active feedback circuit automatically adjusts the current of the current source in response to temperature variations of the MR device. It is also known that air gap variations between the MR device and ferromagnetic materials or objects will affect the resistance of MR devices with larger air gaps producing less resistance and decreased output signals.
Single element magnetic field sensors composed of, for example, an indium antimonide or indium arsenide epitaxial film strip supported on, for example, a monocrystalline elemental semiconductor substrate, are also known. The indium antimonide or indium arsenide film is, for example, either directly on the elemental semiconductor substrate or on an intermediate film that has a higher resistivity than that of silicon. A conductive contact is located at either end of the epitaxial film, and a plurality of metallic (gold) shorting bars are on, and regularly spaced along, the epitaxial film. Examples thereof are exemplified by U.S. Pat. Nos. 5,153,557, 5,184,106 and 5,491,461.
Many kinds of measurements cannot be performed with common magnetic sensors comprising a single sensing element. However, compound semiconductor MRs, such as those manufactured from InSb, InAs, etc. are simply two-terminal resistors with a high magnetic sensitivity and, thus, are very suitable for the construction of single die MR sensors (in most cases one terminal of all the MR elements can be common).
Ultimately, such MR sensors could be integrated on the same die with appropriate processing circuitry. For example, if the MR array was fabricated on a Si substrate then the processing circuitry would be also Si based. For higher operating temperatures, silicon-on-insulator (SOI) could be used. A potentially lower cost alternative to the SOI approach would be to take advantage of the fact that MRs are currently fabricated on GaAs, a high temperature semiconductor, and thus, to fabricate the integrated processing circuitry from GaAs (or related InP) using HBT (Heterojunction Bipolar Tiansistor) or HEMT (High Electron Mobility Transistor) structures. This technology is now easily available and inexpensive through the explosive growth of the cellular phone industry.
Accordingly, what remains needed is a compact and inexpensive die having three magnetic sensing elements and configured to provide a linear position sensor capable of self compensation over wide ranges of temperature and air gaps, including tilts.
The present invention is a compact and inexpensive single die having three MR elements, wherein each MR element thereof is preferably composed of a number of serially connected MR segments.
The present invention is a magnetoresistor linear position sensor incorporated on a single die capable of self compensation over wide temperature ranges and air gaps, including tilts. It employs three MR elements with (preferably) one common bias magnet. The MR sensor is, generally, aligned in the direction of movement of a magnetic target. The middle MR element is the actual linear position sensor. The two outer MR elements serve as reference sensors which sense the magnetic field at the limits of the position sensing range. The cooperating magnetic target assures that one of the two outer MR elements is always exposed to some maximum magnetic field, BMAX, corresponding to a position XMAX, and the other outer MR element is always exposed to some minimum magnetic field, BMIN, corresponding to a position XMIN, and wherein the middle MR element has a portion exposed to BMAX and another portion exposed to BMIN wherein the relative proportion of the portions vary with the position, X, of the target. The effective resistance of the second MR element is proportional to the linear position of the target. Thus, the present invention provides an MR sensor composed of three MR elements for sensing linear displacement of a selected target.
According to a preferred method of fabrication, an indium antimonide epitaxial film is formed, then masked and etched to thereby provide epitaxial mesas characterizing the MR elements. Shorting bars, preferably of gold, are thereupon deposited, wherein the epitaxial mesa not covered by the shorting bars provides the MR segments. The techniques for fabricating epitaxial mesas with shorting bars are elaborated in U.S. Pat. No. 5,153,557, issued Oct. 6, 1992. U.S. Pat. No. 5,184,106, issued Feb. 2, 1993 and U.S. Pat. No. 5,491,461, issued Feb. 13, 1996, each of which being hereby incorporated herein by reference.
Accordingly, it is an object of the present invention to provide an MR die comprising three MR elements capable of detecting one-dimensional position of a magnetic target along an alignment axis of the MR elements.
This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.